Any time a new bond is formed, take a look at the atom that’s accepting the lone pair of

electrons. Does it have a full shell of electrons? Oftentimes it does.

If so, that means you’regoing to have to break a bond that’s attached to that atom. If it’s a single bond you’re

breaking, what you end up creating is called a

leaving group.

Note how the second arrow always shows a pair of electrons going

toward

the leaving group.That means the charge on it is going to become more negative by 1 when it leaves. So if the

leaving group is positively charged, it will become neutral, and if it’s neutral, it will become

negative.The identity of the leaving group is crucial to whether the reaction will happen at all. There

are “good” leaving groups and there are “bad” leaving groups. Reactions are more likely to

take place when you can displace a good leaving group.

What makes a leaving group “good” or “bad”? Thankfully, there’s one simplifying

factor tolook at when deciding this: its

basicity.

Good leaving groups are weak bases.

How do we know what are weak bases? There’s a useful tool for that –

it’s called a pKa table.

Many pKa tables only specifically give you the identity of the acid, but if you think about it,it also gives you information about the

conjugate base

of each acid. The conjugate base is thepart left over when you lose H+.

The stronger the acid, the weaker the conjugate base.

And the weaker the conjugate base, the better the leaving group. So a pKa table is a greatguide to leaving group ability.

One word of caution: pKa measures an equilibrium, whereas leaving group ability is based on

reaction rates. So although the correlation is very good, it isn’t perfect.

Here’s a pKa table, annotated.

The trend is pretty clear

–

in general, the weaker the base, the better the leaving group.

Furthermore, note how we (almost) never see alkanes or hydrogens as leaving groups. That’s because they’re strongly basi

c anions

–

and very unstable.

You might note that I have carefully avoided discussing fluorine. Fluorine tends to be a verypoor leaving group for SN1/SN2/E1/E2 reactions. In Org 2, you may see some exampleswhere F can act as a leaving group when it is attached to a carbonyl carbon or an aromaticring. These reactions (addition-elimination reactions) are a little bit different in that the ratedetermining step is not so related to loss of the leaving group. There are some extra factors atwork in these sit

being associated with better leaving group ability. The correlation is not perfectbecause leaving group ability is a kinetic phenomenon, relating to a reaction's rate, whereasp

K

a

is a thermodynamic phenomenon, describing the position of an equilibrium.Nevertheless, it is a general rule that more highly stabilized anions act as better leavinggroups. Consistent with this rule, strong bases such as alkoxide (RO